The Zn-Ni alloy plated steel sheet, strip or plate exhibits corrosion resistance about five to ten times that of the Zn-plated steel plate having the same deposited amount of the plated metal. For this reason, it is used in an increasing, amount as a steel plate for coping with the problem recently presented of early corrosion of vehicle bodies caused by road freezing preventive agents employed in winter, and is used at various portions from the front fender to a bonnet and trunk lid outer. Also the present-day trend is to use a double side plated steel plate on the outer surface of the vehicle body with the aim of improving the corrosion resistance after peel-off of the coating by the stones or pebbles striking on the outer surface, to replace the one side plated steel plate during the time of early usage of the plated steel plates which was aimed at preventing rusting at the portions where coating may be insufficient, such as the inner surface of the vehicle body.
For preparing the Zn-Ni plated steel plate, there are required properties different from those required of the one side plated plate. Among these properties is the press formability.
In view of the above described usage of the single side plated steel plate, it is a frequent occurrence that the inner side of the punch, that is, the cold rolled surface thereof, turns out to be the protruding surface at the time of press working, so that the plated surface has little effect on the press formability.
However, in the case of the double side plated steel plate, the plated surface turns out to be the protruding surface at the time of press working, and the friction of the plated surface itself presents problems. Although the press working may naturally be facilitated with the use of the highly viscous press oil at the time of press working, the customary practice is to apply the rust preventive oil of low viscosity to the plated steel plate and to perform the press working with the rust preventive oil resting on the plate surface to facilitate the degreasing and coating at the user. The plated plate is required to have satisfactory workability with the rust preventive oil applied thereto.
As the method for improving the press formability of the Zn-Ni alloy plated plate, there is known a method consisting in applying a Zn-Ni alloy plating in double layers to increase the Ni contents of the upper layer, as disclosed in the Japanese Patent Application KOKAI No.141894/1985.
This method utilizes the so-called powdering of the upper Zn-Ni plating layer to improve the press workability, the peeled-off powders are placed and heaped on the press mold surface, in case of continuous stamping of a large number of samples, thus causing the problem of pimples.